Printed substrates include a substrate comprising a laminated sheet and a pattern formed by etching a copper foil stuck onto its surface according to an electrical circuit, said pattern being designed to be mounted thereon with electrical parts such as resistors and capacitors, and a ceramic substrate comprising a ceramic sheet onto which a circuit pattern of silver/palladium is baked. In order to attach electrical parts on such substrates, for instance, their lead terminals are soldered to a soldering land of said circuit pattern.
In this case, such printed substrates are broken down into one type wherein a circuit pattern is formed along with a soldering land on the side of a printed substrate opposite to the side thereof on which electrical parts are to be mounted, and the other, so-called flat packaged type wherein a circuit pattern is formed along with a soldering land on the side of a printed substrate on which electrical parts are also to be mounted. In either case, for soldering,, a molten solder is supplied to the soldering lands and portions of the electrical parts to be soldered. In the former case, however, a jet solder is supplied for soldering. In the latter case, on the other hand, a solder paste is often previously coated on the soldering land to form a film, which is to be molten at the time of soldering and soldered in place.
Such a solder paste consists essentially of a solder powder, a resinous component such as rosin and a solvent, and additionally includes an activator such as amine hydrochloride, a fluidity-affording agent, etc., if required. This solder paste is then coated on a soldering land of a printed substrate by means of screen printing, a dispenser, transfer printing and the like. Thereafter, electrical parts are supplied to the printed substrate, and the paste film is molten and cooled to complete soldering. In order to carry out such soldering, the previously prepared solder paste is delivered to a soldering site where the steps from paste coating to soldering by melting of a solder are usually carried out in a continuous manner.
In such a case, the previously prepared solder paste is not immediately used, and is often used after the lapse of some days, since solder paste makers are not identical with soldering performers. Even when the solder paste is stored in, e.g., a sealed container in the meantime, the solder powder comprising lead, tin, etc. reacts with a vehicle at room temperature, so that its viscosity increases. For instance, the viscosity may double in one week. As the solder powder is oxidized, such an increase in viscosity is further promoted and, eventually, results in a loss of fluidity sufficient for printing, e.g., screen printing.
Such oxidized solder powders are not completely molten when unoxidized solder powders are molten and agglomerated together at the time of solder melting, so that they separate from the agglomerated solder and remain on a coating position, otherwise they are subjected to delayed melting and made to discrete that solder balls are formed. Problems with such solder balls are that they give rise to a short circuit between conductors, a reduction in soldering strength and a degradation of finished appearance.
In order to eliminate such problems with the oxidation of solder powders, a large amount of an activator for reducing solder oxides may be incorporated into solder pastes. For this activator, a hydrochloride of amines may be used. However, a large amount of this substance may accelerate the corrosion of a soldering land, when remaining on the soldering land after soldering. In order to avoid this, sufficient washing should be carried out after the completion of soldering. However, such washing leads to a lowering of working efficiency and cannot completely be carried out as such. Further, the use of such a larger amount of the amine hydrochloride poses a problem that a reaction product of it with solder powders gives rise to an increase in the viscosity of solder pastes.
In order to prevent such a reaction of solder powders with a vehicle or the oxidation of solder powders, solder pastes are stored in refrigerators. However, a problem with this is that when the solder pastes are used, they have to be restored to normal temperature, resulting in a drop of workability.
In the preparation of solder pastes, a solder ingot is finely powderized by, e.g., atomization, immediately followed by kneading with a vehicle. In some cases, however, solder powders are kneaded with a vehicle after the lapse of some time from the preparation of solder powders for consideration of working requirements. In the meantime, the solder powders may be transported. Even in such a case, the solder powders are oxidized upon exposure to air, so that when formed into a solder paste, its viscosity increases markedly, as already mentioned. For that reason, the solder powders are stored in a vacuum, or inert gas atmosphere to avoid oxidation. However, this needs special equipment troublesome to handle. In this respect, improvements have also been desired.